FIG. 8 is a sectional view illustrating the structure of a non-reflection film included in a conventional semiconductor laser device disclosed in Journal of the Optical Society of America, Vol. 51, 1961, pp. 714-718, by J. T. Cox, G. Hass, and G. F. Jacobus. In FIG. 8, the non-reflection film comprises a first film 50 having a refractive index n.sub.1 and a thickness d.sub.1 =.lambda./4n.sub.1 (.lambda.: the oscillation wavelength of the semiconductor laser), a second film 51 having a refractive index n.sub.2 and a thickness d.sub.2 =.lambda./4n.sub.2, and a third film 52 having a refractive index n.sub.3 and a thickness d.sub.3 =.lambda./4n.sub.3. Reference numeral 100 designates a semiconductor laser having an effective refractive index n.sub.c, and numeral 101 designates a facet of the semiconductor laser. The refractive indices n.sub.1, n.sub.2, n.sub.3, and n.sub.c have following relations. EQU n.sub.2.sup.2 n.sub.1 n.sub.3 =n.sub.3 EQU n.sub.c &gt;n.sub.1 &gt;n.sub.2 &gt;n.sub.3 ( 1)
A description is given of the operation. Since the refractive indices of the first to third films 50 to 52 disposed on the laser facet 101 satisfy the formula (1) and the thicknesses thereof are .lambda./4n.sub.i (i=1,2,3), the phase of light traveling through the semiconductor laser 100 toward the facet 101 and the phase of light reflected by the facet 101 are opposite to each other, and the amplitudes thereof are equal to each other. Therefore, no reflection of light occurs at the laser facet 101. The semiconductor laser 100 with the non-reflection film does not oscillate because light generated in an active layer is not repeatedly reflected and amplified in the device.
As described above, when the non-reflection film is disposed on the facet of the semiconductor laser device, the laser oscillation is suppressed, whereby a superluminescent diode emitting incoherent light with a broad spectrum or a semiconductor laser amplifier amplifying incident light is realized.
In the prior art semiconductor optical device, however, in order to achieve the non-reflection film, materials having refractive indices that precisely satisfy the relations of the formula (1) are needed. Therefore, it is very difficult to manufacture the non-reflection film.